Television receivers employing picture enhancement processors are known. There are, for example, processors which provide noise reduction for both luma and chroma, processors which provide vertical and/or horizontal peaking and processors which provide non-linear processing (e.g., so-called "black stretch" or "white stretch" processors), to name but a few.
Typically, picture enhancement processing provides a substantial benefit by improving the subjective appearance of displayed images. However, enhancement processing may produce undesirable effects under certain conditions. One such case is described in U.S. Pat. No. 5,202,765 by Roger L. Lineberry entitled TELEVISION RECEIVER WITH PICTURE IN PICTURE PROCESSING AND NON-LINEAR PROCESSING which issued Apr. 13, 1993. Lineberry found that "black stretch" processing of the main or large picture signal in a picture-in-picture (PIP) television receiver can cause the black level of the small or inset picture to be "modulated" by (i.e., be dependent on) the black level of the main picture. Lineberry addresses the problem by disabling black stretch processing during the small picture interval.
Picture enhancement processing such as peaking can degrade displayed images under certain signal conditions such as when the signal-to-noise ratio is poor or when interference (e.g., impulse noise) is present. For example, in U.S. Pat. No. 4,384,306 by Frank C. Liu entitled VARIABLE PEAKING CONTROL CIRCUIT which issued May 17, 1983, the level of peaking is varied so as to avoid excessive peaking of impulse noise present in the active video or displayed portion of the signal. Also, U.S. Pat. No. 4,376,952 by Troiano entitled NOISE RESPONSIVE AUTOMATIC PEAKING CONTROL APPARATUS which issued Mar. 15, 1983 describes varying peaking as a function of noise by sampling noise during the horizontal blanking interval of a received signal. Another example of varying the level of peaking is video depeaking via AGC control as described in U.S. Pat. No. 3,071,064 by Milbourn entitled VIDEO DE-PEAKING CIRCUIT IN LUMINANCE CHANNEL IN RESPONSE TO AGC SIGNAL which issued Jul. 20, 1976.